1. Field of the Invention
The present invention relates to an information storage disc practiced illustratively as a magnetic disc or optical disc, as well as to an information recording and reproducing apparatus practiced illustratively as a disc drive unit in a mass storage device for use with a data processing system, the disc and the apparatus being used for recording and reproducing information thereto and therefrom.
2. Description of the Related Art
FIG. 1 is a view showing the positional relations of a read head and a write head with respect to the tracks on a disc in a conventional magnetic disc unit. A rotating arm 2 rotating around an arm axis 1 is equipped with the write head W and the read head R. As arrowed in the figure, the magnetic disc 3 rotates counterclockwise. The read head R is located upstream of the write head W.
The tracks on the magnetic disc 3 are divided into a plurality of sectors. Each sector is split into a servo data storage area and a data storage area. At the beginning of each data storage area is an ID storage area in which a sector number and track numbers are first written by use of the write head W. The read head R reads each sector number and track number. If the location read by the read head R is the desired location (i.e., in the desired sector on the desired track), data is written by the write head W to that region in the data storage area which follows the ID storage area.
As illustrated in FIG. 1, the write head W and read head R are arranged to be positioned in an aligned manner on track 4, the innermost track. That is, on the innermost track 4, the radial distance L1 between the write head W and the read head R is very small (close to 0). As a result, when the write head W is positioned on the outermost track 5, the read head R is displaced or dislodged from the track 5 by a distance L2.
FIG. 2 is an enlarged view of part of what is shown in FIG. 1. The write head W and the read head R are shown to be positioned on the innermost track 4, on the outermost track 5 and on an intermediate track 6. Suppose that the write head W is positioned on a given track. In that case, as illustrated, the farther the track on which the write head W is positioned from the disc center, the greater the dislodged distance of the read head R from that track. Conversely, if the read head R is correctly positioned on the target track, the write head W is dislodged from that track correspondingly.
Generally, to write data on the disc requires positioning the write head W at the center of the target track. With the write head W thus positioned, the center of the read head R is bound to be dislodged from the track center. This can result in a failure to read such ID information as sector and track numbers accurately.